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Adiponectin is an adipokine, a type of hormone produced by fat cells which helps to regulate energy metabolism. It makes the body more sensitive to insulin, the hormone that promotes the movement of glucose from the blood into cells, and helps decrease the risk of diabetes and narrowing of the arteries. An adiponectin receptor is a protein found on cells in a number of areas of the body including fatty tissue, the muscles, liver and brain. There are two types of adiponectin receptor, known as AdipoR1 and AdipoR2, to which adiponectin binds, causing changes inside cells. Adiponectin receptor numbers and levels of circulating adiponectin may be altered in association with conditions such as obesity and diabetes.
Research studies carried out in mice have shown that adiponectin lowers levels of glucose in the blood. It also acts against a condition known as insulin resistance, where insulin becomes less effective at moving glucose into cells and preventing the liver from releasing glucose. Insulin resistance is associated with obesity and the development of diabetes and hardened arteries, or atherosclerosis. It is thought that having too much body fat may decrease adiponectin receptor numbers, making cells less responsive to adiponectin, which could lead to insulin resistance. In contrast, weight loss could lead to increases in adiponectin receptor levels, reversing insulin resistance.
The adiponectin receptor AdipoR1 is mainly found in skeletal muscles, while AdipoR2 is mostly present in the liver. In fat cells throughout the human body, AdipoR1 is the type of adiponectin receptor which is most commonly found. Adiponectin exists in two versions, each binding differently to the two receptor types. AdipoR1 is thought to bind extremely well to what is called the globular form of adiponectin, but less well to the full-length version, while AdipoR2 binds moderately well to both forms. The development of drugs which attach to adiponectin receptors and mimic the action of adiponectin, or which increase the numbers of receptors, could provide useful methods of treatment for diseases associated with insulin resistance, such as diabetes.
Growth hormone, which is produced by the pituitary gland, has been found to have an effect on the AdipoR2 adiponectin receptor in particular. Researchers have noted that increased levels of growth hormone are associated with a rise in the number of AdipoR2 receptors in some fat cells. This suggests that growth hormone is involved in changes in insulin sensitivity and the body's energy metabolism.